Formation of microstructures using a preformed photoresist sheet

ABSTRACT

In the formation of microstructures, a preformed sheet of photoresist, such as polymethylmethacrylate (PMMA), which is strain free, may be milled down before or after adherence to a substrate to a desired thickness. The photoresist is patterned by exposure through a mask to radiation, such as X-rays, and developed using a developer to remove the photoresist material which has been rendered susceptible to the developer. Micrometal structures may be formed by electroplating metal into the areas from which the photoresist has been removed. The photoresist itself may form useful microstructures, and can be removed from the substrate by utilizing a release layer between the substrate and the preformed sheet which can be removed by a remover which does not affect the photoresist. Multiple layers of patterned photoresist can be built up to allow complex three dimensional microstructures to be formed.

This invention was made with United States Government support awarded bythe National Science Foundation (NSF), Grant No. EET 8815285. The UnitedStates Government has certain rights in this invention.

This application is a continuation-in-part of application Ser. No.07/994,952, filed Dec. 22, 1992, by Henry Guckel, Todd R. Christenson,and Kenneth Skrobis, entitled "Formation of Microstructures Using aPreformed Photoresist Sheet", the disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor andmicromechanical devices and processing techniques therefor, andparticularly to the photoresist used in formation of microminiaturestructures such as those formed of metal.

BACKGROUND OF THE INVENTION

Deep X-ray lithography involves a substrate which is covered by a thickphotoresist, typically several hundred microns in thickness, which isexposed through a mask by X-rays. X-ray photons are much more energeticthan optical photons, which makes complete exposure of thick photoresistfilms feasible and practical. Furthermore, since X-ray photons are shortwavelength particles, diffraction effects which typically limit devicedimensions to two or three wavelengths of the exposing radiation areabsent for mask dimensions above 0.1 micron. If one adds to this thefact that X-ray photons are absorbed by atomic processes, standing waveproblems, which typically limit exposures of thick photoresists byoptical means, become a nonissue for X-ray exposures. The use of asynchrotron for the X-ray sources yields high flux densities--severalwatts per square centimeter--combined with excellent collimation toproduce thick photoresist exposures without any horizontal runout.Locally exposed patterns should therefore produce vertical photoresistwalls if a developing system with very high selectivity between exposedand unexposed photoresist is available. This requirement has beensatisfied using polymethylmethacrylate (PMMA) as the X-ray photoresist,and an aqueous developing system. See, H. Guckel, et al., "Deep X-rayand UV Lithographies for Micromechanics," Technical Digest, Solid StateSensor and Actuator Workshop, Hilton Head, S.C., Jun. 4-7, 1990, pp.118-122.

Deep X-ray lithography may be combined with electroplating to form highaspect ratio structures. To do so requires that the substrate befurnished with a suitable plating base prior to photoresist application.Commonly, this involves a sputtered film of adhesive metal, such aschromium or titanium, which is followed by a thin film of metal which issuitable for electroplating the metal to be plated. In appropriatecases, the use of an initial layer of adhesive metal is not necessary.Exposure through a suitable mask and development are followed byelectroplating. This process results, after cleanup, in fully attachedmetal structures with very high aspect ratios. Such structures werereported by W. Ehrfeld and co-workers at the Institute for NuclearPhysics (KFK) at Karlsruhe in West Germany. Ehrfeld termed the process"LIGA" based on the first letters for the German words for lithographyand electroplating. A general review of the LIGA process is given in thearticle by W. Ehrfeld, et al., "LIGA Process: Sensor ConstructionTechniques Via X-Ray Lithography," Technical Digest IEEE Solid StateSensor and, Actuator Workshop, 1988, pp. 1-4.

A crucial factor in the production of microminiature devices, such asthose formed by the LIGA process, is the photoresist that is used. Asnoted, PMMA has been successfully used as the photoresist for formationof LIGA structures. The PMMA films have been produced by casting ofliquid MMA directly on the substrate, with the film being reduced to thedesired thickness, generally not more than two to three hundred microns,with a casting jig. The cast film is then solidified, typically byutilizing a polymerization agent or initiator in the casting solutionand a cross-linking agent which results in cross-linking upon curing.There are several disadvantages and limitations of the PMMA films formedin this manner. The casting procedures require special equipment andfixturing, which adds to the time and cost of the process. As withalmost all casting operations, a heat cycle is necessary to produce thesolidified film. Typically, annealing cycles up to 110° C. are required.These heat cycles build up strain in the film due to a significantmismatch in thermal expansion coefficients between the PMMA photoresistand the substrate. Internal strain in the photoresist also occurs due tothe shrinking of the film during curing, which has been observed toresult in up to a 20% shrinkage of the film from its as-cast state. As aconsequence, the cast film, after curing, often has poor adhesion to thesubstrate and can buckle off the substrate. Even where adhesion to thesubstrate is retained, the internal strain that is built into the filmcan result in distortion of the walls formed in the film afterpatterning of the photoresist by X-ray exposure and development.

PMMA photoresist films are typically cross-linked through the additionof a cross-linking agent to the casting solutions to minimize crazing ofthe films. Because of this cross-linking, it is necessary to have anadditional X-ray exposure step, a blanket exposure of the entirephotoresist film, followed by development of the film to remove theresist when its use is complete. Even with the use of cross-linkingagents, the maximum thickness of resists which have been successfullycast, exposed, and developed have been in the range of about 300microns. Most samples of PMMA photoresist films having thickness greaterthan 200 microns have unacceptable amounts of crazing and adhesion loss.Typically, the cast PMMA films may only be used once because such filmsare found to exhibit significant crazing after the microplating of metalinto the patterned openings in the film during the electroplating stepof the LIGA process.

The thicknesses of photoresist utilized for micromechanical processingis typically a few hundred microns or less, which is below the typicalthicknesses of preformed photoresist sheets. Ehrfeld and co-workers havereported attempts to adhere a preformed PMMA photoresist sheet to asubstrate, with the photoresist being calendered before adherence to thesubstrate to reduce the sheet to the desired thickness for carrying outthe LIGA process. However, such attempts were reported to beunsuccessful, apparently because the strain fields in the calenderedphotoresist were excessive.

SUMMARY OF THE INVENTION

In accordance with the present invention, production of microstructuresis facilitated by utilizing a photoresist structure which comprises apreformed sheet which can be adhered to a substrate before furtherprocessing so as to yield essentially no or very little strain withinthe photoresist. The preformed sheet is of conventional thickness, whichallows convenient handling of the sheet as it is adhered to thesubstrate, with the thickness of the sheet being reduced to the desiredthickness for the formation of microstructures by mechanically removinga portion of the sheet, as by milling by a micromill. A preferredphotoresist sheet is formed of substantially non-crosslinked linearpolymethylmethacrylate which has a very high average molecular weightand which is essentially strain free. Either before or after the sheetof photoresist is reduced to the desired thickness, the photoresist maybe patterned by exposure through a mask to radiation which will affectthe susceptibility of the exposed photoresist material to a developer.Depending on the photoresist used, the radiation may be X-ray radiationsuch as from a synchrotron or, in appropriate cases, deep UV light. Theexposed portions of the photoresist (or unexposed portions, depending onthe type of photoresist) may then be removed in a suitable developer.

For carrying out the formation of electroplated microstructuresutilizing the present invention, a plating base is applied to asubstrate prior to the photoresist. A non-exposed photoresist may thenbe adhered onto the plating base, and the photoresist then exposed. Theexposed portions are then removed using a developer, and metal is thenelectroplated onto the exposed plating base to fill the area defined bythe void(s) in the photoresist. The remaining photoresist may then beremoved. Where the photoresist is formed of non-crosslinked PMMA,removal can take place by utilizing a solvent which dissolves thenon-crosslinked PMMA. Where a cross-linked PMMA sheet is utilized, anadditional blanket exposure to synchrotron X-rays is required before thephotoresist is removed. By utilizing the preferred non-crosslinked PMMAphotoresist sheet, this additional exposure step can be avoided, whichproduces a significant savings in time and expense over procedures whichrequire a blanket exposure.

Where the photoresist is to be applied over existing structures formedon a substrate, relatively thin coats of conventional photoresist, suchas PMMA, may then be spun onto the substrate to cover the structures.The photoresist sheet is then placed on top of the spun on photoresistand the interface between the two is wetted with a monomer. For example,with high molecular weight PMMA being utilized as the material of thephotoresist sheet, a lower molecular weight PMMA dissolved in a solventis spun on to the substrate to cover the mechanical structures, and themonomer, liquid methylmethacrylate, is then applied to the interfacebetween the two which results in a solvent bonding of the materials atthe interface. The preformed photoresist sheet is generally much thickerthan the spun on photoresist, which need only be thick enough to coverthe existing structures, typically a few microns or tens of microns inheight.

The adherence of a preformed photoresist layer onto the substrate, andthe mechanical milling of the photoresist sheet to a desired thickness,results in a photoresist of any desired thickness which can be preciselycontrolled using the mechanical milling process. Good adhesion of thephotoresist sheet to the substrate is obtained and with very lowinternal strain within the photoresist. Consequently, much thickerphotoresist structures can be formed than has heretofore been possibleutilizing conventional photoresist materials without substantialdistortion of the walls of the photoresist during the process ofexposing the photoresist and removing the exposed resist with developer.The photoresist of the invention may be utilized during more than oneelectro-deposition process, inasmuch as the preformed PMMA sheetexhibits much less crazing and other damage during the electro-platingprocess than is observed in conventional cast PMMA photoresist layers.

The present invention may further be used to produce photoresiststructures which can be released from the substrate. The strain freephotoresist sheet, e.g., PMMA, either linear or cross-linked, is adheredto a release layer on a substrate. After the photoresist sheet ispatterned, the release layer may be removed by a remover which etches ordissolves the release layer without substantially affecting theremaining photoresist. Because the photoresist sheet is strain free, thephotoresist parts which are thus freed from the substrate aredimensionally stable and will not distort or curl.

The present invention also allows multiple layers of patternedphotoresist to be constructed which can be applied to a substrate orutilized as a separate product. Such multiple layer structures allow theformation of metal structures by electrodeposition which can have avariety of shapes which vary in all three dimensions. For example, themetal structures may be formed having upper portions which are wider orwhich extend outwardly from the lower portions, and which undergoseveral variations in geometry from the top to the bottom of thestructure. Such multiple layer photoresist structures can be formed invarious manners in accordance with the present invention. In anexemplary multiple layer process, a second layer of photoresist isbonded to the upper surface of the first layer after the first layer hasbeen exposed but before it has been developed. The second layer is thenmilled to the desired thickness and an X-ray exposure of the secondlayer photoresist takes place. The exposed photoresist is then developed(removed) and metal structures may be electroplated in the voids vacatedby the exposed photoresist. Structures having more than two layers maybe built up in this manner. Generally, it is necessary when utilizingthis process that the exposure for the second layer (or subsequentlayers) either lie within the exposed regions in the first layer (or allunderlying layers) or that the incident X-rays be sufficient to fullypenetrate and expose all layers of photoresist.

In another exemplary multilayer process, a layer of preformedphotoresist (e.g. linear or cross-linked PMMA) which is not adhered to asubstrate is exposed on one side. The photoresist sheet may be quitethick, e.g. 1 mm to 3 mm, so that the incident X-rays do not necessarilycause sufficient exposure of the photoresist in all exposed areas toallow the photoresist sheet to be fully exposed all the way through. Theexposed photoresist may then be developed with a developer to remove it,and then the preformed sheet may be bonded to a substrate on the sidewhich had been exposed and developed. The free side of the photoresistis then milled down to a desired thickness which is below the level atwhich all of the areas exposed to X-rays were sufficiently exposed sothat they will be completely removed by the photoresist to leave apattern of voids therein. This provides an initial single layerstructure which is equivalent to a structure produced by bonding aphotoresist to a substrate first, and then a milling to a desiredthickness and then exposing to X-rays and developing the exposedphotoresist. However, multiple photoresist layers may be formed bytreating a free sheet (unbonded to a substrate) of photoresist in thismanner, whether the initial layer is formed in this manner or is formedafter being adhered to the substrate. For example, the second sheet ofphotoresist may be exposed to X-rays on one side in a desired pattern,the exposed photoresist developed to remove it, and then the surface ofthe photoresist which has been exposed may be bonded to the free surfaceof the first layer, with milling of the free surface of the second layercarried out to reduce the thickness of the second layer to below thelevel at which the exposed photoresist has been developed and fullyremoved to leave a pattern of voids. Third and additional layers may beformed in a similar manner.

The preformed photoresist sheet may also be exposed on one side and afirst layer bonded to a substrate (or to a previously appliedphotoresist layer) at the surface which had been exposed to X-rays, butwithout developing the exposed photoresist. The sheet is then milleddown to a level which is below the level at which the exposedphotoresist would be fully removed from the developer. The photoresistmay then be removed immediately. However, it is not necessary to do so,and a second preformed photoresist sheet may be treated in a similarmanner, exposing one side to X-rays partially through, adhering theexposed side to the prior layer, and then milling down the second layerto a thickness such that all areas of the photoresist which were exposedto X-rays are sufficiently fully exposed so that they will be fullydeveloped. When the desired number of layers have been built up, theentire laminated structure may be developed with a liquid photoresistdeveloper to remove all of the exposed photoresist. In this process, itis necessary that the areas of exposed photoresist in each layer overlapone another so that the developer can work through the layers to removeall of the exposed photoresist. Alternatively, the exposed regions mustbe accessible at the side edges of the laminate.

In either of the above-processes, it is not essential that the substratebe a metallic substrate. For example, the substrate may comprise a thickphotoresist sheet, or a variety of other materials, including asemiconductor wafer with or without electronic circuitry thereon. Thisinitial thick photoresist sheet may also have been previously processedso that it contains either developed structures or undeveloped X-rayexposed regions. Additional photoresist layers are then bonded to thefirst layer and milled in sequence to form a laminate, with the variouslayers either being developed before or after they are bonded together.The initial thick photoresist sheet which had acted as the substrate maythen be milled at its exposed side to reduce it to a desired thicknesswhich exposes either the developed photoresist in the sheet or the fullydeveloped structures. The entire multilayer laminate may then be bondedto a metal substrate for electrodeposition, or the laminate may beutilized itself as a structural component. For example, the undevelopedphotoresist structure may be formed in desired three dimensional shapeswhich can be utilized for various purposes, for example as molds, or asstructural components. Multilayer laminates can include, for example,multiple channels which can be used to provide conduits for liquid orgases, or conductive paths.

Where the photoresist sheet or laminate is applied to a semiconductor(e.g., single crystal silicon) substrate having microcircuits formedthereon, the sheet or laminate may be fully exposed, and developed ifdesired, as described above, before being adhered to the substrate. Inthis manner, no radiation need be applied to the photoresist when it ison the substrate which could damage microcircuits on the substrate.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a simplified illustrative side view of a substrate withexisting micromechanical structures formed thereon.

FIG. 2 is a view of the substrate of FIG. 1 with a coating ofphotoresist thereon.

FIG. 3 is a view of the substrate of FIG. 2 with the addition of apreformed sheet of photoresist on top of the initial layer ofphotoresist.

FIG. 4 is a view of the substrate of FIG. 3 illustrating the applicationof a monomer at the interface between the preformed sheet and theinitial layer of photoresist, with a weight pressing the preformed sheetto the underlying layer.

FIG. 5 is a view of the substrate of FIG. 4 after the preformed sheet ismilled down to a desired thickness.

FIG. 6 is an illustrative view showing the photoresist and substrate ofFIG. 5 exposed through an X-ray mask to X-ray radiation.

FIG. 7 is an illustrative view showing the substrate and photoresist ofFIG. 6 after the exposed photoresist has been developed.

FIG. 8 is an illustrative view of the substrate of FIG. 7 afterelectroplating of metal into the areas from which the photoresist hasbeen removed.

FIG. 9 is an illustrative view of the substrate with the micromechanicalparts formed thereon after the remaining photoresist has been strippedfrom the substrate.

FIG. 10 is a simplified illustrative side view of a substrate with asacrificial layer formed thereon.

FIG. 11 is a view of the substrate of FIG. 10 with a strain freephotoresist sheet adhered to the sacrificial layer and thus to thesubstrate.

FIG. 12 is an illustrative view showing the photoresist and substrate ofFIG. 11 exposed through an X-ray mask to X-ray radiation.

FIG. 13 is an illustrative view showing the substrate and photoresist ofFIG. 12 after the exposed photoresist has been developed.

FIG. 14 is a perspective view of an exemplary free part formed ofphotoresist material which may be formed in accordance with theinvention.

FIG. 15 is an illustrative side view of a substrate having a preformedphotoresist sheet thereon which has been exposed and machined but withthe exposed regions not yet developed.

FIG. 16 is an illustrative view of the substrate of FIG. 15 after asecond preformed photoresist layer is bonded onto the first layer andmachined to a desired thickness.

FIG. 17 is an illustrative view showing the substrate and photoresistlayers of FIG. 16 exposed through an X-ray mask to X-ray radiation.

FIG. 18 is an illustrative view of the substrate and photoresist of FIG.17 after the exposed photoresist has been developed.

FIG. 19 is an illustrative side view of a relatively thick preformedsheet of photoresist exposed through an X-ray mask to X-ray radiation.

FIG. 20 is an illustrative view showing the photoresist sheet of FIG.19, after exposure and development of the exposed photoresist, which ispositioned to have its exposed surface bonded to a previously developedphotoresist sheet on a substrate.

FIG. 21 is an illustrative view showing the free photoresist sheetbonded to an underlying layer of photoresist with alignment of thedeveloped regions in the two layers of photoresist.

FIG. 22 is an illustrative view of the substrate and photoresist layersof FIG. 21 after machining of the top layer down to a thickness suchthat all of the developed regions in the second photoresist sheet areexposed.

FIG. 23 is an illustrative view of the substrate and photoresist layersof FIG. 22 with the addition of a third layer of photoresist which maybe formed in the same manner as illustrated above in FIGS. 19-22.

FIG. 24 is an illustrative side view of a relatively thick preformedphotoresist sheet exposed through an X-ray mask to X-ray radiation.

FIG. 25 is an illustrative side view of the exposed but not developedphotoresist sheet of FIG. 24 which is bonded to a layer of exposed butnot developed photoresist on a substrate.

FIG. 26 is an illustrative side view of the substrate and photoresistlayers of FIG. 25 after the top layer of photoresist is machined down toa thickness such that all of the fully exposed regions of photoresist inthe top layer are exposed.

FIG. 27 is an illustrative view of the substrate with photoresist layersof FIG. 26 after development and removal of the exposed photoresist.

FIG. 28 is an illustrative side view of a relatively thick preformedphotoresist sheet exposed through an X-ray mask to X-ray radiation.

FIG. 29 is an illustrative view of two relatively thick preformedphotoresist sheets which are exposed to X-rays in the manner illustratedin FIG. 28 and which have been developed to remove the exposedphotoresist.

FIG. 30 is an illustrative view showing the two layers of preformed,exposed and developed photoresist sheet of FIG. 29 bonded together attheir exposed surfaces.

FIG. 31 is an illustrative view of the photoresist layers of FIG. 30after the top layer has been machined down to a thickness which fullyexposes the developed regions of the top photoresist layer.

FIG. 32 is an illustrative view of the photoresist layers of FIG. 31with the addition of another photoresist layer which is formed in themanner illustrated above with respect to FIGS. 28-31.

FIG. 33 is an illustrative view of the multilayer photoresist laminateof FIG. 32 with the free surface of the top layer bonded to a substrateand before machining down of the thick photoresist layer.

FIG. 34 is an illustrative side view of portions of a photoresist layer,on a substrate to illustrate the formation of alignment holes in thephotoresist layer.

FIG. 35 is an illustrative view of the substrate with photoresiststructure of FIG. 34 with alignment pegs inserted in the alignmentholes.

FIG. 36 is an illustrative side view of a relatively thick preformedphotoresist sheet which has been exposed on one surface to X-rayradiation and developed to remove exposed photoresist in variousregions, including regions which are sized and positioned to constitutealignment holes.

FIG. 37 is an illustrative view showing the preformed photoresist sheetof FIG. 36 drawn into alignment with the structure of FIG. 35 byinsertion of the alignment pegs in the alignment holes of the preformedphotoresist sheet.

FIG. 38 is an illustrative view of the substrate and photoresist layersof FIG. 37 after machining of the top layer of photoresist down to athickness which exposes the alignment holes and the other exposedregions in the top layer.

FIG. 39 is an illustrative view of the insertion of additional alignmentpegs into the alignment holes left in the top layer of the photoresistsheet of FIG. 38.

FIG. 40 is an illustrative view of the structure of FIG. 39 with theaddition of a third layer of photoresist sheet and a third set ofalignment pegs which are formed in the same manner as described above.

FIG. 41 is an illustrative view of the structure of FIG. 40 with metalelectroplated into the open regions left in the multiple layers ofphotoresist of the structure of FIG. 40.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be utilized in the formation ofmicrostructures as carried out in LIGA processes and extensions thereof.Exemplary processes for carrying out the production of micromechanicalmetal structures is described in U.S. patent application Ser. No.07/874,116 by Henry Guckel, filed Apr. 24, 1992, now U.S. Pat. No.5,190,637, entitled Formation of Microstructures by Multiple Level DeepX-Ray Lithography With Sacrificial Metal Layers, the disclosure of whichis incorporated herein by reference. The present invention may also beutilized for producing patterned photoresist which may be used for otherapplications, as where the photoresist is used directly for purposesunrelated to providing a mold for the electro-deposition of metal. Theprocess allows the formation of thick photoresist structures which enjoysubstantially no internal strain after adherence to the substrate, goodadhesion to the substrate, and generally ready removability from thesubstrate without substantial additional processing, with photoresistthicknesses up to 1,000 microns being readily achievable. Films castdirectly onto a substrate in comparable thicknesses inevitablyexperience internal strain which detracts from the integrity of thefilms, increases the likelihood of poor adhesion to the substrate, andleads to crazing of the films. The present invention may be utilizedwith relatively thin layers of spun on film as part of a multiple layerstructure, as described below, with the spun on film thicknessestypically being in the range of 5 μm or less, and in multilayerstructures formed from two or more preformed photoresist sheets.

In the present invention, a preformed photoresist sheet, preferably oneformed of linear (non-cross linked), high molecular weight (>2 million)polymethylmethacrylate (PMMA), may be adhered to a substrate before orafter patterning in a manner which firmly bonds the preformed sheet tothe substrate without introducing internal strain in the substrate. Thepreformed sheet may be commercially obtained, and generally will havelittle or no internal strain associated with it in its free form. Asused herein, a strain free preformed photoresist will not substantiallymechanically distort after portions of the photoresist are exposed andremoved or when the remaining photoresist is released from thesubstrate. The preformed sheet is adhered to the surface of the chosensubstrate utilizing a suitable adhesive. In the present invention, apreferred adhesive layer for use with a preformed PMMA sheet is arelatively thin spun on film of PMMA which is applied to the substratein a conventional manner and cured and forms a strong bond to thesubstrate. Because the initial spun on film of PMMA is relatively thin(1 μm to 5 μm or less), the initial spun on film is not subject to theadhesion problems, internal strain problems, and crazing which iscommonly associated with the much thicker (25 μm to 200 μm) films whichare utilized in LIGA processing. Because the initial PMMA film layerwill be covered by the preformed PMMA sheet, and is relatively thin, theinitial PMMA layer will not suffer crazing during the electro-depositionprocess and does not need to have a cross-linking agent added to it tomaintain the integrity of the film. Consequently, by using anon-crosslinked preformed PMMA sheet of high molecular weight PMMA, theentire photoresist may be readily removed using a suitable solvent afterthe patterning and deposition of the metal have occurred, withoutrequiring a blanket exposure to X-rays or UV of the entire surface ofthe substrate on which the photoresist film is formed. This eliminates atime consuming and relatively expensive final step which has beenheretofore an essential step in LIGA processing. Adhesion between thepreformed PMMA sheet and the initial PMMA layer may be readily obtainedby utilizing methylmethacrylate (MMA) monomer liquid applied to theinterface between the preformed sheet and the initial layer. The monomerwicks through the interface between the two layers by capillary action,causing a bond between the preformed sheet and the initial layer, and isreleased by diffusion through the preformed sheet after a sufficientperiod of time. The preformed sheet adhered to the substrate may bemechanically worked, as by milling, to reduce the thickness of the layerto a precise height above the surface of the substrate, e.g., by usingcommercially available micromilling equipment.

After the photoresist has been applied and milled to the desiredthickness, it may be patterned in a conventional manner, such as byusing X-ray radiation from a synchrotron and an X-ray mask, to renderareas of the photoresist susceptible to removal by a developer liquid.High molecular weight, linear PMMA exhibits good selectivity toconventional developers so that the unexposed PMMA is not substantiallyaffected by the developer, while the exposed PMMA, which consists ofPMMA molecules of substantially shorter length as a result of molecularchain scission from the radiation exposure, is readily dissolved by thedeveloper.

The process can further be extended to the formation of additionalmicrostructures on a substrate on which relatively thin microstructureshave already been formed. The basic process and the extended process mayboth be illustrated with respect to the views of FIGS. 1-9. Withreference to FIG. 1, a substrate 11, e.g., single crystal silicon,glass, quartz, etc., may have previously formed microstructures 12thereon. These microstructures will typically be in the range of a fewμm in thickness (e.g., 2.5 μm). In typical LIGA processing, a metalplating base 13 is formed on the top surface of the substrate 11 toprovide a base for the structures 12 which are deposited byelectroplating onto the plating base 13. The next step, as illustratedin FIG. 2, is the spinning on of a layer of liquid linear PMMA, e.g.,496 k molecular weight PMMA in a layer 15 on the surface of thesubstrate 11 (over the plating base 13) and over the microstructures 12.

As illustrated in FIG. 3, a preformed photoresist sheet 17 of linearPMMA is then placed on top of the spun-on layer 15 of PMMA. The exposedinterface 18 between the layer 15 and the preformed sheet 17 is thenwetted with the monomer, in this case MMA, and the sheet 17 is pressedagainst the layer 15 in a suitable manner, for example, using a piece ofaluminum foil 20, a layer of steel wool 21 and a mass 22 to provide acontrolled, evenly distributed pressure to the preformed sheet 17. Aftera suitable period of time, the weights 21 and 22 and the foil 20 areremoved, and the monomer is allowed to diffuse through the preformedsheet 17. Thereafter, the substrate is placed in a micromill and thelayer 17 is milled down to a desired thickness at a selected heightabove the top surface of the substrate 11.

With reference to FIG. 6, the sheet 17 and the layer 15 are exposed in apattern to X-rays 24 through an X-ray mask 25 having an X-ray absorber26 thereon in a pattern. The X-rays, or in appropriate cases, UVradiation, which is passed by the mask is incident on areas 27 in thephotoresist 17 and the underlying layer 15. These areas 27 experiencechain scission of the high molecular weight material therein and thusbecome susceptible to removal by a developer. The resulting structure isillustrated in FIG. 7 where the exposed areas have been removed by adeveloper to leave openings 29 in the layer 17, some of which bottom outon the plating base 13 on the surface of the substrate 11, and others ofwhich may expose all or part of the original microstructures 12 whichwere on the substrate. As illustrated in FIG. 8, metal is thenelectroplated into the openings 29 in a conventional manner to providemetal structures 30 on the substrate in addition to the original metalstructures 12. The final step involves the removal of all the remainingphotoresist, both the initial PMMA layer 15 as well as the remainder ofthe preformed sheet 17, to leave the free standing metal structures 12and 30 on the surface of the substrate 11. The free standing structures30 may be quite high, since they can be as high as the thickness of themilled photoresist sheet 17. Exemplary structures may be formed to aheight of 200 μm to 300 ∞m or greater, allowing structures to be formedat thicknesses which were not heretofore readily obtainable usingconventional LIGA processing techniques.

An X-ray sensitive preformed photoresist layer of 50 μm to 1000 μm orgreater can be prepared on a variety of substrates. Because the presentinvention requires no heat cycles once the resist is attached, thethermal expansion mismatch between the thick resist layer and thesubstrate is negligible. The resulting resist layer has very goodadhesion and very low internal stress.

Exemplary processing procedures in accordance with the invention aredescribed below. These procedures use the following materials: adhesionpromotor, APX-K1, from Brewer Science, Inc., P.O. Box GG, Rolla, Mo.65401; liquid photoresist, 496 k PMMA photoresist, from OLIN HUNT/OCG, 3Garret Mountain Plaza, West Paterson, N.J. 07424, comprisingpolymethylmethacrylate with molecular weight of 496,000 dissolved inchlorobenzene (9 wt %); preformed polymethylmethacrylate sheet, fromGoodfellow, Suite 140, 301 Lindenwood Drive, Malvern, Pa. 19355-1758,comprising polymethylmethacrylate (PMMA) with weight average molecularweight greater than 2 million (M_(w) /M_(n) approximately 2-3), notappreciably crosslinked; methyl methacrylate monomer, from AldrichChemical Company, Inc., 1001 West Saint Paul Avenue, Milwaukee, Wis.53233.

Procedure for Preparing Thick PMMA Resist Layer on Smooth MetalizedSurface

An initial step involves preparation of the substrate for formation ofmicromechanical structures. A typical substrate is a Si wafer, coveredwith about 1 μm Si0₂. A metal plating base is then applied. A typicalbase includes a first layer of 200 Å Ti, DC sputtered, followed by asecond layer of 200 Å Ni, DC sputtered. The substrate is then removedfrom the sputter system, and the adhesion promoter, APX-K1, is spun-onat 3000 rpm, for 30 seconds, followed by a 30 second hotplate bake at145° C. The use of the adhesion promotor is optional, and may be omittedin many applications. A layer of 496 k PMMA (9 wt %) is then spun-on at2000 rpm for 60 seconds. The wafer is then subjected to a bake/annealcycle: 60° C./hr to 180° C., 180° C. for 1 hour, then 60° C./hr to roomtemp.

The preformed PMMA sheet (e.g., 1/2"×3/4") is then placed near thecenter of substrate. Using a micropipet, 10 μl of methyl methacrylate isintroduced to the edge of the sheet to be glued-down. The interface iswet with the methyl methacrylate as a result of capillary action.

The sample is then covered with aluminum foil and lightly weighted usinga 1/2" thick piece of steel wool (wrapped with aluminum foil) and aweight of approximately 1 kg. The sample should remain covered for atleast 1 hour, after which the weights and aluminum foil can be removedand the sample stored in a vented wafer box (preferably N₂ purged).

To avoid cracking of the film during or shortly after milling, themonomer should be allowed to diffuse and evaporate from the film for aminimum of 8 hrs. Following the 8 hour drying period, the sample may bemilled to the desired thickness. The PMMA photoresist formed in theforegoing manner is both deep UV and X-ray sensitive, and is compatiblewith the remaining steps of conventional LIGA processing. The thickresist layer may be removed through immersion in methylene chloride.

Procedure for Preparing Thick PMMA Resist Layer on Non-Planar Substrate

For substrates having existing structures thereon, the following is anexemplary process. A typical substrate may be a 3" Si wafer with metalstructures of about 2.5 μm step heights.

First, the adhesion promoter, APX-K1, is optionally spun-on at, e.g.,3000 rpm, for 30 seconds, followed by a 145° C., 30 second hotplatebake. Then, 496 k PMMA (9 wt %) is spun-on, with a 10 second delay priorto spinning, at 2000 rpm, for 30 seconds. A bake/anneal cycle follows:60° C./hr to 180° C., 180° C. for 1 hour, then 60° C./hr to roomtemperature. Another 496 k PMMA (9 wt %) layer is spun-on, with a 10 secdelay prior to spinning, at 2000 rpm, for 30 seconds. A bake/annealcycle as above follows.

The preformed photoresist sheet is then adhered to the 496 k PMMA layeras described above. Preformed photoresist sheets with thicknesses of 1mm are commercially available and readily handled. After being adheredto the substrate as described above, the preformed layer may be milledto a desired thickness (to within ±2.5 μm) with surface finish of lessthan ±0.1 μm, using a fly cutter with either a diamond or a CBN (cubicboron nitride) tool. An example of a suitable milling machine is aJung/Reichert polycut E ultramiller. The resist layer obtained has verygood adhesion and appears to have very low internal stress. Removal ofthis thick resist does not require a blanket exposure. Since the resistis composed of linear polymethylmethacrylate, removal may beaccomplished through dissolution in a suitable solvent. To preventexcessive swelling and cracking of the resist, which could damageadjacent, delicate metal structures, methylene chloride is generallysuitable as the solvent. To clear 200 μm to 300 μm of resist, the sampleis immersed in 200 ml of CH₂ Cl₂ for 20 to 30 minutes, then again into100 ml CH₂ Cl₂ to rinse. An oxygen plasma may also be employed as afinal clean.

Procedure For Preparing Released PMMA Photoresist Structures

Free structures formed of photoresist, such as PMMA, may also be formedin accordance with the present invention. The process for producing suchstructures is illustrated with respect to FIGS. 10-14. Referringinitially to FIG. 10, the process begins with the application of a thinsacrificial layer 40 onto a substrate 41. The substrate typically has aplanar surface on which the sacrificial layer 40 is formed, and may bemade of a variety of materials, e.g., a silicon wafer, glass, metal, orvarious plastics. The material of the sacrificial layer may be any of avariety of materials which is resistant to attack from the photoresistdeveloper. For example, where the photoresist is PMMA, the sacrificiallayer of material must be resistant to a typical PMMA developer such asmorpholine, 2-(2-butoxyethoxy) ethanol, ethanolamine, and water. Thesacrificial layer of material must also be selectively removable by aremover which does not attack the photoresist, e.g., PMMA. For a PMMAphotoresist, examples of suitable sacrificial layers are titanium(sputtered on to the substrate), which can be removed with dilutehydrofluoric acid, and partially imidized polyimide (which is spun onthe substrate), with a suitable remover for the polyimide being ammoniumhydroxide. A soft PIRL polyimide material available from Brewer Sciencecan be utilized as the sacrificial layer, as described in the aforesaidU.S. Pat. No. 5,190,637, the disclosure of which is incorporated hereinby reference.

As illustrated in FIG. 10, the preformed strain-free photoresist sheet,for example, the preferred PMMA sheet described above, is then adheredas a layer 42 to the sacrificial layer 40 in the manner described above.The layer may be milled down as described above to reach a desiredthickness less than the initial sheet thickness.

As illustrated in FIG. 12, an X-ray mask 44 having X-ray absorbingpatterns 45 formed thereon provides a pattern exposure from synchrotronradiation X-rays 46 to provide exposed patterns 47 in the photoresistsheet 42. The exposed photoresist is then developed using a highselectivity developer, as described above, to remove the exposedphotoresist 47, leaving, in the exemplary structure of FIG. 13, isolatedphotoresist structures 48 adhered to the sacrificial layer 40. A removerof the sacrificial layer is then applied to the sacrificial layer in thesubstrate to selectively etch away the sacrificial layer 40, therebyfreeing the structures 48 from the substrate. An example is shown inFIG. 14 of a free part which has the form of a lens, which, as shown inFIG. 14, has a body with sidewalls substantially perpendicular to thetop and bottom surfaces of the body. By using the processes describedabove, these structures may be formed with at least one of the top orbottom surfaces of the body being milled, and with the thickness of thebody between the top and bottom surfaces typically being less than about1 mm. Where PMMA is utilized as the photoresist, a structure formed inthis manner may have useful optical properties inasmuch as the PMMA is asubstantially transparent material and thus is well suited for use as alens. Of course, many other types of structures may be formed in thismanner which can be separated from the substrate. In addition, the PMMAstructures 48 remaining on the sacrificial layer may be interconnected,having patterned openings therein, which, when removed from thesacrificial layer, may be utilized as molds for molding of other parts.A separated PMMA sheet with holes of desired size formed therein mayalso be utilized as a filter or sieve having desired flow propertiesthrough the openings formed in this manner. If desired, the initial PMMAsheet may be cross-linked so that the completed parts are also ofcross-linked PMMA.

Because the preformed photoresist sheet is substantially strain-free,when the parts 48 are freed from the substrate upon removal of thesacrificial layer, the parts have substantially no internal straintherein and will not substantially mechanically distort or curl, as iscommonly the case with photoresist materials, such as PMMA, which areapplied in liquid form to a substrate and cured to solidify thephotoresist.

Formation of Multilayer Photoresist Structures

In accordance with the present invention, multilayer photoresiststructures comprising laminates of two or more preformed photoresistsheets, or a cast layer and one or more preformed sheets, can beproduced for use either as forms for producing three dimensionalmetallic parts of a desired geometry, or for their own utility asstructural components. Such structures are obtained utilizing theforegoing procedures in accordance with the invention. A variety ofprocess sequences are possible to obtain such multilayer structures.

In a first exemplary process sequence, illustrated in FIGS. 15-18, asubstrate 50 has a layer 51 of photoresist formed thereon which hasexposed regions 52. The photoresist layer 51 may constitute a preformedphotoresist sheet exposed and machined as described above. However, ifdesired, the initial layer 51 may be formed in a more conventionalmanner as a cast layer of, e.g., PMMA, which is cured and cross-linkedand exposed in a pattern to X-rays. Next, as illustrated in FIG. 15, apreformed photoresist sheet 54 (e.g., linear PMMA as described above) isthen bonded to the first layer 51 and machined down to a desiredthickness. As illustrated in FIG. 17, X-rays 55 are passed through anX-ray mask 56 having X-ray absorbing material 57 in a pattern thereon toexpose regions 58 in the second layer 54. The regions 58 may lieentirely within underlying areas 52 so that only these previouslyexposed regions 52 receive X-rays during the exposure of the secondlayer 54. Alternatively, the thicknesses of the layers 51 and 54, andthe length and intensity of the exposure of the X-rays 55, may beselected so that X-rays passed through the mask 56 penetrate entirelythrough both the layers 54 and 51 to fully expose the regions underlyingthe open areas of the mask 56 in both of the layers 51 and 54. Thelatter case is illustrated in FIG. 17 where the X-rays 55 exposeadditional regions of the first layer 51 beyond the original exposedregions 52. Of course, it is understood that the mask 56 is twodimensional, and there may be and generally will be some areas of theregions 52 which will not receive any X-ray exposure from the X-rays 55and some areas of the first layer 51 where the X-rays 55 will not extendbeyond the previously exposed regions 52.

The foregoing process can be repeated for as many layers as desired eventhough only two are shown in FIG. 17. When exposure of the last layer iscompleted, the photoresist in the exposed regions can then be removed byapplying liquid developer to the photoresist to leave void regions 60into which metal may be electroplated in the manner described above. Themultiple layers 51 and 54 which form a laminate on the substrate 50 maythen be removed, if desired, in the manner described above. Generally,the regions 58 in the second layer (or subsequent layers) should overlapat least in part all underlying void regions 52 in the first layer 51 sothat the developer liquid can reach the regions 52 in the first layer.When all of the exposed photoresist is removed, voids 60 are left in themultilayer laminate.

Each of the layers 51 and 54 (and additional layers) may be formed froma typical commercially available photoresist sheet, which is commonly inthe 1 mm to 3 mm thickness range, formed of a high molecular weightX-ray sensitive material e.g., PMMA as described above. The bonding of apreformed sheet to an underlying surface of a photoresist sheet may beaccomplished by solvent bonding as described above, using methylmethacrylate (MMA) which wets the interface between the two adjoiningsheets, but other bonding techniques which do not substantially damagethe structural integrity of the photoresist sheets are also acceptable.Where the photoresist sheets are linear or relatively low molecularweight PMMA, the MMA monomer will generally provide adequate solventbonding of the two sheets. If a highly cross-linked PMMA sheet is to beadhered on an underlying cross-linked PMMA sheet, a procedure asdescribed above for adhering a PMMA sheet to a general substrate may beused. For example, a layer of relatively low molecular weight PMMA(e.g., 496 k PMMA) in a solvent may be spun on in a thin layer andcured. The MMA monomer can act on the low molecular weight PMMA layer toprovide solvent bonding between that layer and the highly cross-linkedPMMA sheet. The low molecular weight PMMA layer is also sensitive toX-ray exposure and will be patterned in the same way as the preformedPMMA sheet.

Another multiple layer process sequence is illustrated in FIGS. 19-23.As illustrated in FIG. 19, a fairly thick sheet of preformed photoresist62 (e.g., 1 mm to 3 mm PMMA) is exposed to X-rays 63 through an X-raymask 64 having X-ray absorber 65 formed thereon in a pattern. Theregions 66 which are exposed to the X-rays passed through the mask 64receive a sufficient X-ray exposure to render these areas susceptible toa photoresist developer. However, in this case, the photoresist sheet 62is sufficiently thick that the regions 66, which are sufficientlyexposed to the X-rays so as to render these areas susceptible to adeveloper, extend only partially through the thickness of thephotoresist sheet 62. For example, the depth of the region 66 may onlybe a few hundred microns or less where the entire sheet 62 may have athickness of 1,000 microns (1 mm) or more.

After the preformed photoresist sheet 62 is exposed to X-rays, the sheethas a liquid developer applied to it which removes the photoresist inthe regions 66 to leave open regions 67, as illustrated in FIG. 20. Thepreformed sheet 62 is then joined with a previously formed layer 69 ofphotoresist having developed areas 70, with the layer 69 being mountedon a substrate 71. The layer 69 can be formed on the substrate 71 in anyof the various ways described above (e.g., casting liquid PMMA or theuse of a preformed photoresist). The preformed photoresist sheet 62 isthen aligned with the underlying photoresist layer 69 and bonded to thesurface of that layer so that the void regions 67 in the second layerpreformed sheet 62 align with the void regions 70 in the first layer 69.Thereafter, as illustrated in FIG. 22, the photoresist sheet 62 ismachined down to a thickness which is less than the height of theregions 67 above the surface of the sheet 62, fully opening up theregions 67 and allowing communication to the underlying regions 70 inthe first photoresist layer 69. The foregoing process can be repeated asmany times as desired to build up a multilayer laminate, as illustratedin FIG. 23 in which a third photoresist layer 73 having open regions 74is bonded to the second photoresist layer 62. The geometry of theopenings in the laminate, defined by the open regions 70, 67 and 74 inthe three layers, can be relatively arbitrary. These regions can then befilled with metal by electrodeposition as described above. When themultiple layers 73, 62 and 69 are removed (e.g., with a blanketdeveloper as described above), metal structures remain on the substrate71 which can have a variety of three dimensional shapes of substantiallyarbitrary geometry.

It should also be understood that in the process of FIGS. 19-23, thepreformed and developed sheet 62 may be adhered as described above toany other suitable substrate; i.e., the substrate 71 and first layer 69effectively comprise a "substrate" to which subsequent layers ofphotoresist may be adhered. The process is general. For example, thesubstrate may comprise a wafer of silicon on which microcircuits areformed in conventional semiconductor processing techniques. Thedeveloped photoresist sheet 62 may be adhered to the silicon substrateto cover the circuitry thereon. Because the photoresist sheet 62 hasbeen exposed to X-rays separately before it is attached to thesubstrate, no X-ray exposure of the substrate is required and damage tocircuitry on the substrate is avoided. The layer 62 may be machined downto expose the voids 67, which can allow access to portions of thecircuitry thereunder. For example, the voids 67 may be chosen to alignto electrical connecting pads for an integrated circuit on thesubstrate, with conducting metal being electroplated into the voids toprovide upright connections to these conducting pads. If desired, thephotoresist may then be removed as described above, or it may be left onthe integrated circuit to protect it. PMMA is an advantageous materialfor such purposes since it is generally transparent and may be used tocover optically active devices on the substrate.

In a variation of the foregoing process sequence, a relatively thicksheet of photoresist 80 is exposed to X-rays 81 through an X-ray mask 82having X-ray absorber material 83 in a pattern thereon to define regions85 in the side of the photoresist sheet 80 which are exposed to X-raysand which are susceptible to being removed by a developer, asillustrated in FIG. 24. However, rather than removing the photoresist inthe regions 85 immediately by applying a developer, the photoresistsheet is adhered at its exposed side to a first photoresist layer 87,having regions 88 therein that have been exposed to X-rays and aresusceptible to a developer, with the first layer 87 being adhered to asubstrate 89, as illustrated in FIG. 25. Thence, as shown in FIG. 26,the second layer 80 is machined down until it is of a thickness suchthat the X-ray exposed regions 85 are reached and fully exposed at thefree surface of the layer 80. This process may be repeated to form aphotoresist laminate of as many layers as desired. After the desirednumber of layers is reached, the entire structure is exposed to adeveloper which removes all of the exposed photoresist. This isillustrated in FIG. 27 for the two layer structure in which thephotoresist has been removed from the regions 88 in the first layer 87to leave open regions 90, and from the regions 85 in the second layer 80to leave for open regions 91. The regions 85 and 88 generally must becontact with each other to allow the developer liquid to reach theregions 88, or, alternatively, the regions 88 must be exposed at a sideedge of the laminated structure so that the developer can reach theseregions from this side edge. Generally, it is preferred that the regions85 and 88 overlap each other to provide good removal of the exposedphotoresist by the developer. If the structure illustrated in FIG. 27 isnot the final product that is desired, but rather it is desired to useit as a mold to form metal structures, metal can be electroplated intothe open regions 90 and 91 in the manner described above, and the layers80 and 87 of photoresist can then be removed to leave the metalstructures on the substrate.

Either of the two foregoing process sequences can be carried oututilizing a preformed photoresist sheet having X-ray exposed areastherein as the substrate. The preformed photoresist sheets in normalthicknesses (1-3 mm, or even greater as desired) are sufficiently strongand have sufficient structural integrity to allow such sheets to be usedas substrates on which one or more layers of photoresist are formed andmachined. An exemplary process sequence (similar to the sequence ofFIGS. 19-23) is shown in FIGS. 28-33. However, it should be understoodthat an essentially identical process can be carried out using thesequence of steps as in FIGS. 24-27, wherein the exposed photoresist isremoved after all of the photoresist layers have been adhered togetherin the laminate. Referring to FIG. 28, a relatively thick preformedphotoresist sheet 100 is exposed to X-rays 101 passed through an X-raymask 102 having X-ray absorber 103 thereon in patterns which result inregions 105 in the preformed sheet 100 which are sufficiently exposed toX-rays to be removed by developer, but with the regions 105 extendingonly part way through the thickness of the photoresist sheet 100. Thephotoresist sheet 100 is then exposed to a liquid developer whichremoves the exposed photoresist in the regions 105. Another photoresistsheet 108 is formed by a similar process to have void regions 109therein. The two relatively thick photoresist sheets 100 and 108 arethen bonded together at their exposed surfaces in a properly alignedmanner so that the void regions 106 and 109 properly align with eachother, as illustrated in FIG. 30. The layer 100 may then be machineddown to a thickness wherein the regions 106 are fully exposed, asillustrated in FIG. 31. A further photoresist sheet 110 having openregions 111 therein may be formed on the two layers 100 and 108 in anentirely identical manner. After the desired number of layers are formedin the laminate, the laminate may, if desired, then be bonded to asubstrate 113 which has a surface thereon appropriate forelectrodeposition of metal, as illustrated in FIG. 33. The now top layerphotoresist sheet 108 may then be machined down to a thickness whichexposes the open regions 109, allowing electrodeposition of metal intoall of the regions 109, 106 and 111.

However, it should be understood that the laminate of layers 108, 100and 110 of photoresist may itself have independent utility without beingbonded to a substrate as a mold for electrodeposition. For example, theopen regions 106, 109 and 111 may comprise multiple fluid channels toallow routing of liquids or gases through the laminate for use inpressure sensors, alarm devices, hydraulic or pneumatic actuators, etc.The channels formed by such open regions may be sealed off in thelaminate structure of FIG. 32 by simply attaching another layer ofphotoresist to the exposed surface of the laminate. In essence, such astructure is shown in FIG. 33 in which the substrate 113 may simplyserve the purpose of closing off the open regions 111, and may, ifdesired, be formed of any of a variety of materials, including apreformed photoresist sheet, or metal, or ceramic materials. Further, itis seen that the structure of FIG. 30 allows multiple channel structuresto be formed with only two layers without machining of either layer whenproperly joined and aligned so that the open regions 106 and 109 areproperly positioned with respect to each other.

Although the invention has been exemplified above with PMMA sheets, apositive photoresist, it is understood that the invention can also becarried out using a negative photoresist in which exposure to radiationrenders the material less susceptible to a developer.

The joining together of two or more separately formed layers ofphotoresist, which have either void regions or regions exposed to X-raystherein, such that these regions are in proper alignment to each other,requires reasonably precise alignment of each layer. Generally, it isdesired to be able to maintain tolerances to a micron or less. The useof a relatively thick photoresist can contribute to large alignmentgaps. Relatively precise alignment and clamping procedures are required.Commercially available equipment for optical alignment of X-ray masksmay be utilized to align the multiple layers, but such equipment isrelatively expensive.

Alignment of the various exposed photoresist layers can be obtained bycreating mechanical alignment structures during exposure of each layerand then using these alignment structures to obtain mechanicalregistration between each layer and a subsequent layer. Exemplaryalignment structures can consist of relatively large (e.g., 1 mmdiameter) holes on opposite sides of the exposure area which can acceptpegs which may be formed of a photoresist material itself (e.g., PMMA)or metal. Alignment is established by assembling the subsequent layeronto the pegs and gluing of the photoresist sheet to the underlyinglayer. Self-alignment is obtained because the alignment holes areexposed at the same time as the desired pattern in the layer, andtherefore the alignment tolerance is governed by assembly tolerances.

The alignment procedure is illustrated in the views of FIGS. 34-41. Withreference to FIG. 34, a substrate 120 is shown having a first layer 121of photoresist formed on it (in any manner) which has alignment openings122 formed therein (e.g., cylindrical openings) and void regions 123which constitute the desired structural openings. The exposure andmasking processes that are carried out to form the openings 122 and 123are as described above so that the openings 122 and 123 are formed atthe same time; therefore, the relatively position of the openings 122with respect to the openings 123 is precisely controlled. The substrate120 may have a metallic plating base if desired.

Alignment pegs 125 are then inserted in the openings 122 as illustratedin FIG. 35. These pegs can be formed of metal by various techniques(they are relatively large, e.g., 1 mm in diameter) or the pegs can bemade of the material of the photoresist, such as PMMA, which may beformed as free structures in the manner as described above with respectto FIGS. 12-14. A second layer of preformed photoresist sheet 131 isthen formed as shown in FIG. 36, having alignment openings 132 andstructural void regions 133 therein which extend partially through, butnot necessarily entirely through, the relatively thick (e.g., 1 mm PMMAfilm) photoresist sheet. The second layer photoresist sheet 131 is thenaligned with the first sheet by inserting the portions of the pegs 125that protrude above the exposed surface of the first layer 122 into thealignment openings 132 in the second layer 131. Again, the control ofthe relative position of the alignment openings 132 to the structuralvoid regions 133 insures that the structure area 133 is now properlyaligned with the structural void regions 123 in the first layer 121.

As illustrated in FIG. 38, the photoresist sheet 131 may then bemachined down to a thickness such that the void regions 133 and thealignment openings 132 are fully exposed. However, the reduction inthickness of the second layer 131 preferably does not reduce it to athickness such that the pegs 125 are exposed. Rather, a portion of eachof the alignment openings 132 remains open, as illustrated in FIG. 38.Into the remaining portion of these alignment openings 132, a second setof alignment pegs 135 is inserted, as illustrated in FIG. 39. Then, asillustrated in FIG. 40, a third layer 141 of photoresist sheet can thenbe aligned, mounted and machined down in the same manner as the secondlayer 131 to provide a three layer laminate. This results in thestructural void regions 143 in the third layer 141 being in a properalignment with the structural void regions 133 in the second layer 131,and the structural void regions 123 in the first layer 121. A third setof pegs 145 may then be inserted in the alignment opening holes formedin the third layer 141 in the same manner as described above and furtherlayers added in a similar manner.

After the desired number of layers have been built up in the laminate,metal can be electroplated into the structural void regions, if desired.This is illustrated in FIG. 41, in which an electroplated metal piece150 is formed which has three sections, 151, 152 and 153, whichcorrespond to the structural void regions in the layers 121, 131 and141, respectively. If the alignment pegs 125 and 135 left in thelaminate are formed of photoresist, they too can be removed in the samemanner as the photoresist material 121, 131, and 141. If the pegs areformed of metal, they generally will be released from the substrate whenthe surrounding photoresist layers are removed.

Alternatively, the alignment pegs 125 at the first stage can befabricated of metal and attached permanently to the substrate. Thissubstrate can then be used multiple times if a completely free metal orphotoresist (e.g., PMMA) structures are desired. If such metal pegs areutilized, the first PMMA layer may be exposed and developed separatelyand then adhered to the substrate surface in proper alignment byinsertion of the pegs 125 into the alignment holes formed in the layer.

It is also possible to utilize variations of the foregoing processeswhich provide development of the exposed regions in the photoresistlayers after formation of the laminate. This can be done by partialdevelopment so that only the PMMA exposed at the positions of thealignment holes are removed, for example, by careful masking of the PMMAlayers. Alternatively, the photoresist sheets may have alignment holesmachined therein and then be exposed to X-rays under the X-ray maskwhich is aligned to the holes which had been machined into thephotoresist sheet.

By utilizing the foregoing procedures, alignment tolerances of plus orminus 1 micron or less can be obtained while reducing the amount of timerequired to align the various layers.

It is understood that the invention is not confined to the particularembodiments set forth herein as illustrative, but embraces all suchmodified forms thereof as come within the scope of the following claims.

What is claimed is:
 1. A method of forming microstructures comprisingthe steps of:(a) providing a preformed sheet of photoresist materialwhich can be exposed to radiation to affect its susceptibility to adeveloper; (b) exposing the photoresist sheet in a pattern to radiationwhich will change its susceptibility to a developer; (c) mechanicallyremoving the material of the photoresist sheet to reduce the thicknessof the sheet to a desired thickness; and (d) applying a developer to theexposed photoresist to remove photoresist which is susceptible to thedeveloper.
 2. The method of claim 1 including the initial steps ofapplying a thin layer of metal to a subtrate as a plating base, adheringthe preformed sheet of photoresist to the plating base, then after allof the steps of claim 1, the additional steps of electroplating metal inthe areas in which the photoresist has been removed, and then removingthe remaining photoresist to leave the electroplated metal structures onthe substrate.
 3. The method of claim 1 wherein the photoresist sheet isPMMA.
 4. The method of claim 3 wherein the PMMA of the photoresist sheetis very high molecular weight linear PMMA.
 5. The method of claim 2wherein the material of the photoresist sheet is very high molecularweight linear PMMA, and wherein the step of removing the remainingphotoresist is carried out by exposing the photoresist to a bath of asolvent which dissolves the PMMA.
 6. The method of claim 5 wherein thesolvent which is used to dissolve the PMMA is methylene chloride.
 7. Themethod of claim 3 wherein the step of exposing the photoresist in apattern is carried out by exposing the photoresist to X-rays from asynchrotron.
 8. The method of claim 4 including the steps of spinning ona thin initial layer of non-crosslinked PMMA onto the surface of asubstrate, solidifying the initial layer of PMMA, applying the preformedsheet of PMMA to the initial layer, and applying a liquidmethylmethacrylate monomer to the interface between the preformed sheetand the initial layer to provide adhesion between the preformed sheetand the initial layer at the interface between the two.
 9. The method ofclaim 1 including the initial steps of applying a sacrificial layer to asubstrate and adhering the preformed sheet to the sacrificial layer,then after all of the steps of claim 1, the additional step of removingthe sacrificial layer with a remover which does not affect thephotoresist material to free the remaining photoresist from thesubstrate.
 10. The method of claim 9 wherein the material of thephotoresist is PMMA.
 11. The method of claim 10 wherein the sacrificiallayer is a material selected from the group consisting of titanium andpartially imidized polyimide.
 12. The method of claim 10 wherein thePMMA of the photoresist is a very high molecular weight linear PMMA. 13.The method of claim 10 wherein the PMMA of the photoresist iscrosslinked PMMA.
 14. The method of claim 1 wherein the preformed sheetof photoresist is at least 1 mm thick, and in the step of mechanicallyremoving the material of the photoresist sheet the thickness of thesheet is reduced to less than 1 mm.
 15. A method of formingmicrostructures comprising the steps of:(a) providing a first preformedsheet of photoresist material which can be exposed to radiation toaffect its susceptibility to a developer; (b) exposing the photoresistsheet to radiation in a pattern to which will change its susceptibilityto a developer; (c) mechanically removing the material of thephotoresist sheet to reduce the thickness of the sheet to a desiredthickness constituting a first layer of photoresist; (d) providing asecond preformed sheet of photoresist material which may be exposed toradiation to affect its susceptibility to a developer; (e) adhering thesecond photoresist sheet to the first layer of photoresist to form alaminate; (f) mechanically removing the material of the secondphotoresist sheet to reduce the thickness of the sheet to a desiredthickness constituting a second layer of photoresist; (g) exposing thefirst and second layers of the photoresist to a pattern of radiationwhich will change the susceptibility of the photoresist of the secondlayer to a developer; and (h) removing the photoresist in the first andsecond layers which is susceptible to a developer using a developer forthe photoresist.
 16. The method of claim 15 including the step ofadhering the first preformed sheet of photoresist material to asubstrate before the steps of exposing the first photoresist sheet to apattern of radiation and mechanically removing the material of the firstphotoresist sheet.
 17. The method of claim 16 wherein the step ofadhering the first sheet of photoresist to the substrate includes thestep of applying a thin layer of metal to the substrate as a platingbase and adhering the preformed sheet of the photoresist to the platingbase, then after all of the steps of claim 16, the additional steps ofelectroplating metal in the areas in which the photoresist has beenremoved, and then removing the remaining photoresist to leave theelectroplated metal structures on the substrate.
 18. The method of claim15 wherein the first and second photoresist sheets are formed of PMMA.19. The method of claim 18 wherein the PMMA of the photoresist sheets isvery high molecular weight linear PMMA.
 20. The method of claim 17wherein the material of the photoresist sheets is very high molecularweight linear PMMA, and wherein the step of removing the remainingphotoresist is carried out by exposing the photoresist to a bath of asolvent which dissolves the PMMA.
 21. The method of claim 15 wherein thesteps of exposing the photoresist sheets in a pattern is carried out byexposing each photoresist sheet to X-rays from a synchrotron.
 22. Themethod of claim 15 including the additional steps of providing a thirdpreformed sheet of photoresist material which can be exposed toradiation to change its susceptibility to a developer;adhering the thirdphotoresist sheet to the second layer of the laminate; mechanicallyremoving the material of the third photoresist sheet adhered to thesecond layer to reduce the thickness of the sheet to a desired thicknesscomprising a third layer of the laminate; exposing the third layer ofphotoresist in a pattern to radiation which will change thesusceptibility of the photoresist to a developer, and then carrying outthe step of removing the photoresist susceptible to a developer in eachof the first, second and third layers utilizing a developer for thephotoresist.
 23. The method of claim 15 wherein the step of removing thephotoresist susceptible to a developer in the first and second layers iscarried out before the second photoresist sheet constituting the secondlayer is adhered to the first layer of photoresist.
 24. A method offorming microstructures comprising the steps of:(a) providing a firstpreformed sheet of photoresist material which can be exposed toradiation to affect it susceptibility to a developer; (b) exposing thefirst photoresist sheet to radiation in a pattern which will result insusceptibility of the photoresist to a developer partially but notentirely through the thickness of the photoresist sheet; (c) adheringthe first photoresist sheet on the side which has photoresist which issusceptible to a developer to a layer of photoresist which has a patternof areas therein which are susceptible to the developer, to form alaminate; (d) mechanically removing the material of the first preformedphotoresist sheet to reduce the thickness of the sheet to expose theareas of the first preformed sheet which are susceptible to removal bythe developer; and (e) applying a developer to the laminate to removephotoresist which is susceptible to the developer to leave a pattern ofvoids in the first photoresist sheet and the underlying photoresistlayer.
 25. The method of claim 24 including, before the step of adheringthe first photoresist sheet to the photoresist layer, the additionalsteps of providing the photoresist layer from a preformed sheet ofphotoresist material which can be exposed to radiation to affect itssusceptibility to a developer partially but not entirely through thethickness of the photoresist sheet, mechanically removing the materialof the photoresist sheet from the side opposite the exposed side of thesheet to reduce the thickness of the sheet to fully expose the areas ofthe sheet which are susceptible to the developer to provide thephotoresist layer, and thereafter adhering the first preformedphotoresist sheet to the photoresist layer.
 26. The method of claim 25including before the step of mechanically removing the material of thepreformed photoresist sheet from which the photoresist layer is formed,the step of adhering the exposed side of that preformed photoresistsheet to a substrate, and then carrying out the step of mechanicallyremoving the material of the preformed photoresist sheet.
 27. The methodof claim 24 wherein the photoresist layer and the preformed photoresistsheet are formed of PMMA.
 28. The method of claim 26 wherein the step ofadhering the photoresist layer to the substrate includes the step ofapplying a thin layer of metal to the substrate as a plating base andadhering the photoresist layer to the plating base, and then after allof the steps of claim 26 wherein the areas susceptible to developer inthe photoresist have been removed to leave voids, the additional stepsof electroplating metal in the voids in which the photoresist has beenremoved, and then removing the remaining photoresist to leave theelectroplated metal structures on the substrate.
 29. The method of claim26 wherein the first preformed photoresist sheet and the photoresistlayer are formed of very high molecular weight linear PMMA.
 30. A methodof forming microstructures comprising the steps of:(a) providing a firstpreformed sheet of PMMA photoresist material; (b) exposing the firstphotoresist sheet in a pattern to X-rays to render exposed areas of thephotoresist sheet susceptible to a developer; (c) mechanically removingthe material of the first photoresist sheet to reduce the thickness ofthe sheet to a desired thickness; and (d) applying a developer to thefirst photoresist sheet to remove the exposed photoresist which issusceptible to the developer.
 31. The method of claim 30 including theinitial steps of applying a thin layer of metal to a substrate as aplating base, adhering the first preformed sheet of photoresist to theplating base, then after all of the steps of claim 30, the additionalsteps of electroplating metal in the areas in which the photoresist hasbeen removed, and then removing the remaining photoresist to leave theelectroplated metal structures on the substrate.
 32. The method of claim31 wherein the material of the first photoresist sheet is very highmolecular weight linear PMMA, and wherein the step of removing theremaining photoresist is carried out by exposing the photoresist to abath of a solvent which dissolves the PMMA.
 33. The method of claim 32wherein the solvent which is used to dissolve the PMMA is methylenechloride.
 34. The method of claim 30 including the steps of spinning ona thin initial layer of non-crosslinked PMMA onto the surface of asubstrate and solidifying the initial layer of PMMA, applying the firstpreformed sheet of PMMA to the initial layer, and applying a liquidmethyl methacrylate monomer to the interface between the preformed sheetand the initial layer to provide adhesion between the preformed sheetand the initial layer at the interface between the two.
 35. The methodof claim 30 wherein the first preformed sheet of photoresist is at least1 mm thick, and in the step of mechanically removing the material of thephotoresist sheet the thickness of the sheet is reduced to less than 1mm.
 36. The method of claim 30 further comprising the steps of:(a)providing a second preformed sheet of PMMA photoresist material; (b)adhering the second photoresist sheet to the first photoresist sheet toform a laminate; (c) mechanically removing the material of the secondphotoresist sheet to reduce the thickness of the sheet to a desiredthickness; (d) exposing the second photoresist sheet to a pattern ofX-rays which will render the exposed areas of the second photoresistsheet susceptible to a developer; and (e) removing the exposedphotoresist in the second photoresist sheet using a developer for thephotoresist.
 37. The method of claim 36 including the step of adheringthe first preformed photoresist sheet to a substrate before the steps ofexposing the first photoresist sheet to a pattern of radiation andmechanically removing the material of the first photoresist sheet. 38.The method of claim 37 wherein the step of adhering the first sheet ofphotoresist to the substrate includes the step of applying a thin layerof metal to the substrate as a plating base and adhering the preformedsheet of the photoresist to the plating base, then after all of thesteps of claim 37, the additional steps of electroplating metal in theareas in which the photoresist has been removed, and then removing theremaining photoresist to leave the electroplated metal structures on thesubstrate.
 39. The method of claim 38 wherein the material of thephotoresist sheets is very high molecular weight linear PMMA, andwherein the step of removing the remaining photoresist is carried out byexposing the photoresist to a bath of a solvent which dissolves thePMMA.
 40. The method of claim 36 including the additional steps ofproviding a third preformed sheet of PMMA photoresist;adhering the thirdphotoresist sheet to the second photoresist sheet of the laminate;mechanically removing the material of the third photoresist sheetadhered to the second photoresist sheet to reduce the thickness of thesheet to a desired thickness; exposing the third sheet of photoresist ina pattern to X-rays which will render the exposed areas of thephotoresist susceptible to a developer, and then carrying out the stepof removing the exposed photoresist in each of the first, second andthird photoresist sheets utilizing a developer for the photoresist. 41.The method of claim 36 wherein the step of removing the exposedphotoresist with a developer in the first and second photoresist sheetsis carried out before the second photoresist sheet is adhered to thefirst photoresist sheet.
 42. The method of claim 36 wherein the step ofadhering the second photoresist sheet to the first photoresist sheet iscarried out after the steps of exposing the second photoresist sheet toa pattern of X-rays and of removing the exposed photoresist.
 43. Themethod of claim 36 wherein the step of adhering the second photoresistsheet to the first photoresist sheet is carried out before the step ofapplying a developer to the first photoresist sheet.
 44. The method ofclaim 36 wherein the second photoresist sheet is adhered to the firstphotoresist sheet by wetting the interface between the sheets withmethyl methacrylate.